Engines may be operated with boosted aircharge provided via a turbocharger wherein an intake compressor is driven by an exhaust turbine. However, placing a turbine in an exhaust system can increase engine cold-start emissions due to the turbine acting as a heat sink. In particular, during the engine cold-start, engine exhaust heat may be absorbed at the turbine, lowering the amount of exhaust heat that is received at a downstream exhaust catalyst. As such, this delays catalyst light-off. Consequently, spark retard may be required in order to activate the exhaust catalyst. The fuel penalty associated with the spark retard usage may offset or even outweigh the fuel economy benefit of the boosted engine operation. Also during the cold-start, due to lower than threshold cylinder wall and piston temperatures, there may be an increase in hydrocarbon emissions. Therefore, heat may need to be supplied to the engine to increase the temperature of the cylinder wall and piston.
Accordingly, various approaches have been developed to expedite the attainment of a catalyst light-off temperature during cold-start conditions in a boosted engine. One example approach, shown by Andrews in U.S. Pat. No. 8,23,4865, involves routing exhaust gas towards an exhaust tailpipe via a passage that bypasses the exhaust turbine during cold-start conditions. A passive, thermatically operated valve is used to regulate the flow of exhaust through the passage, the valve opening during low-temperature conditions (such as during cold-start). The thermatically operated valve comprises a bi-metallic element which distorts based on temperature thereby regulating the opening of the valve. By circumventing the turbine, exhaust heat may be directly delivered to the exhaust catalyst.
However, the inventors herein have recognized potential issues with such systems. As one example, heat from the exhaust may not be sufficiently recovered and used for expedited heating of the cylinder walls and piston. Due to the resulting lower cylinder temperature, hydrocarbon emissions may increase. Also, due to the exhaust bypassing the turbine, there may be a delay in turbine spin-up, resulting in turbo-lag and reduced boost performance. Furthermore, after catalyst light-off, the temperature of the unobstructed exhaust reaching the catalyst may be higher than desired. Owing to a coating on the catalyst surface (such as on the surface of an exhaust oxidation catalyst or three-way catalyst), the catalyst may have higher conversion efficiencies at lower exhaust temperatures. The higher than desired temperature of exhaust reaching the catalyst may result in reduced catalyst functionality.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for a boosted engine comprises: during cold-start, flowing exhaust first through a three-way catalyst, then an underbody converter, then an exhaust bypass passage with a heat exchanger, and then a turbine; transferring heat from exhaust to coolant circulating through the heat exchanger; and heating an engine cylinder and piston with exhaust heat recovered at the heat exchanger. In this way, exhaust heat may be used to increase cylinder wall and piston temperatures, and also expedite catalyst light-off.
In one example, a turbocharged engine system may be configured with a branched exhaust assembly wherein the exhaust passage, downstream of the exhaust manifold, is divided into at least three separate branches, each creating a distinct flow path. The branches may be interconnected to each other via valves such that an order of exhaust flow along each of the flow paths can be adjusted via adjustments to a position of the valves. Distinct exhaust components may be coupled to distinct branches of the branched exhaust assembly. For example, an exhaust turbine of the turbocharger may be coupled to a first branch, an underbody converter may be coupled to a second branch, and an exhaust oxidation catalyst (three-way catalyst) may be coupled to a third branch of the exhaust assembly. An exhaust bypass passage may also couple an engine coolant system to the first branch, upstream of the turbocharger, such that exhaust received in the first branch may be directly routed from upstream of the turbine into the bypass passage, without passing through the turbine. A heat exchanger may be coupled in the bypass passage wherein heat from the exhaust may be transferred to a coolant circulating through the heat exchanger. Adjustments to the position of a diverter valve may be used to control exhaust flow via the bypass passage. During cold-start conditions, a position of the exhaust system valves may be adjusted by an engine controller to flow exhaust first through the catalyst, then through the underbody converter, then through the bypass passage, and then through the turbine. Heat extracted from the exhaust by the coolant circulating through the heat exchanger may be used to increase cylinder wall and piston temperatures. After catalyst light-off, a position of the valves may be adjusted to flow exhaust first through the turbine, then through the underbody converter, and then through the catalyst. This allows for expedited turbine spin-up. During high engine load conditions, such as while operating with boost, a position of the valves may be adjusted such that exhaust may be simultaneously routed to the tailpipe through two separate flow paths. For example, a first portion of the exhaust may first flow through the bypass passage, then through the turbine, then through the underbody converter, and then through the light-off catalyst before exiting via the tail pipe. A second (remaining) portion of exhaust may flow first through turbine, then through the underbody converter, and then through the light-off catalyst before exiting via the tail pipe. The portion of the exhaust routed through the heat exchanger may be adjusted based on engine heating demands and engine load. Also during high engine load conditions, a cooling fluid may be injected into the exhaust flow upstream of the turbine to reduce the temperature of the exhaust entering the turbine.
In this way, by routing exhaust through different flow-paths of a branched exhaust assembly, it is possible to expedite attainment of catalyst light-off temperature while concurrently extracting exhaust heat, and providing boost to the engine during cold-start conditions. Specifically, exhaust can be flowed through each of a heat exchanger, a turbine, an exhaust catalyst, and an underbody converter, with an order of exhaust flow through the components adjusted based on operating conditions. By adjusting exhaust flow during cold-start conditions to route hot exhaust first through an exhaust catalyst, and then through a heat exchanger before flowing the exhaust through the remaining exhaust components, exhaust heat may be effectively used for heating catalyst, and other engine components. By adjusting the exhaust flow after catalyst activation to route the hot exhaust through an exhaust turbine before flowing the exhaust through the remaining exhaust components, turbo lag is reduced. In addition, a temperature of the exhaust received at the catalyst is lowered, improving catalyst conversion efficiency. The technical effect of recovering exhaust heat using an engine coolant is that heating of cylinder walls, and piston may be expedited, and hydrocarbon emissions may be reduced, especially during cold-start conditions. By routing exhaust via multiple flow paths in the exhaust assembly, it is possible to lower the temperature of exhaust flowing through the turbine thereby reducing the possibility of boost error, and turbine hardware malfunction during high engine load conditions. Also, by using a cooling liquid upstream of the turbine to reduce exhaust temperature, damage to turbine hardware may be reduced. Overall, by changing an order of exhaust flow through exhaust components, and recovering exhaust heat, engine efficiency, emissions quality, and fuel efficiency may be improved in a boosted engine system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.